This invention relates to power supply systems.
The invention is more particularly, but not exclusively, concerned with power supply systems for aircraft.
In an aircraft a.c. power supply system there is only a limited amount of power available from the aircraft generators. Inductive loads such as fans and pumps tend to draw high currents when first switched on, before settling to a lower steady value. When several of these loads are turned on at the same time, large and damaging currents can be drawn unless measures are taken to avoid this. One way of reducing the risk of damage is to increase the generator capacity but this leads to a consequent increase in the size, weight and expense of the generators. Alternatively, the system can be arranged to stagger the starting of large loads, and in particular, large inductive loads, such as hydraulic pumps.
Conventional systems have fixed time delay relays associated with the highly inductive loads. Each time delay is made different, with the highest priority loads having the shortest delays. In this way, if all the loads are commanded on simultaneously, the most important loads will be started first and the less important loads will be started after longer intervals. The problem with this arrangement, however, is that, if a low priority load is commanded on by itself, there will be a relatively long and unnecessary delay before power is supplied to the load. Also, the arrangement does not entirely remove the risk that power could be supplied to several large inductive loads at the same time. For example, three hydraulic pumps A, B and C could be given different delays so that if switches controlling the operation of the three pumps were switched on at the same time, power would be supplied first to pump A, then to pump B and finally to pump C. However, if the switches controlling the pumps were turned on separately, one after the other, in the order C, B, A, this would reduce the time delay between power being supplied to the pumps and could lead to overloading of the generators.
Also, when the power source is only lightly loaded, it may not be necessary to stagger the switching of loads. If the system has lost one power source it will usually be necessary to stagger the application of loads onto the remaining power source, whereas, when both sources are operating, this may not be necessary. Current systems, however, are not sufficiently adaptable and cause staggering of the loads even when this may not be necessary.